WASHINGTON, D.C. — The Senate Thursday triggered the so-called “nuclear option” that allowed Republicans to break a Democratic filibuster of Supreme Court nominee Neil Gorsuch. The Senate is now expected to vote to confirm Gorsuch Friday.

The controversial changes to Senate rules, made along partisan lines, allows filibusters of Supreme Court picks to be broken with only 51 votes rather than 60.

The action Thursday and Friday caps more than a year of tension over an empty Supreme Court seat, as both parties in the Senate are poised to take action leading to an outcome neither party wants.

It’s a situation loaded with nuance, procedural twists and Senate history — not to mention a spot on the nation’s highest court — and a standoff that reflects a peak in polarization following a deeply divisive presidential election.

Triggering the nuclear option came after Senate Democrats Thursday morning blocked the nomination of Gorsuch under the previous 60-vote threshold.

Both sides are crying foul.

Democrats still want Republicans to pick another nominee.

Republicans want Democrats to show more bipartisanship and allow Gorsuch to advance through the process.

Leading up to the vote, lawmakers on both sides lamented the change, saying it could lead to even more partisan animosity down the road, forever changing a historic element of the Senate.

Republican Sen. Susan Collins of Maine was heavily involved in the bipartisan conversations to find a compromise, but she said Wednesday they were simply unsuccessful.

“I’ve had midnight calls on this, 6:30 a.m. calls on this,” she said. “Worked all weekend and we just couldn’t get here.”

A Democratic aide in the Senate, who asked not to be named to speak more freely, called the impasse a “damn shame.”

“There’s been plenty of appetite from both sides to find a way to avoid this, and even hope that we might at times, but it looks like that’s over,” the aide said. “This is happening. And it’s a damn shame. This hurts both parties in the long term because it hurts the institution.”

What’s happening?

A filibuster over a Supreme Court nominee isn’t the same as the hours-on-end speechmaking theatrics, a la “Mr. Smith Goes To Washington.”

Rather, it happens when senators force a nominee to clear the 60-vote threshold for cloture, a technical term for breaking a filibuster of the nomination. It’s the same threshold used for legislation, and it’s a unique element of the Senate, compared to the House, that requires at least some bipartisan support to advance to a final vote.

Republicans hold a 52-48 majority in the Senate. So to get around the 60-vote obstacle, they plan to use the unprecedented “nuclear option,” a change in Senate rules that would lower that 60 vote threshold for Supreme Court nominees down to a simple majority of 51.

Senate Majority Leader Mitch McConnell will take a series of procedural steps to position the Senate to re-vote on that same motion to break the filibuster.

A majority vote is needed to approve these motions. Democrats could take other steps — like multiple parliamentary inquiries and possibly other roll call votes — to try to stall the nomination the Senate.

They can’t prevent a vote on Gorsuch, however.

Once McConnell has finally positioned to re-vote on breaking the filibuster, he could make a point of order that it should take 51 votes instead of 60 to overcome a filibuster of a Supreme Court nominee — this is the nuclear option.

There will be a roll call vote to affirm that position, and a majority vote is needed for this to pass.

Not all GOP senators have confirmed they will support the nuclear option — such as Sen. Bob Corker and Collins — and Republicans can’t afford to lose more than three of their own.

So all eyes will be on this vote. Vice President Mike Pence is supposed to be available to break a tie if needed.

Once the nuclear option is completed, the Senate will cast a second vote to break the filibuster at the new lower 51-vote threshold.

Next will come 30 hours of additional debate time, per Senate rules, before a final vote on Gorsuch’s confirmation, which is expected sometime Friday evening.

How did this all come about?

On January 31, Trump announced Gorsuch as his pick for the next Supreme Court justice, filling a seat vacated when Antonin Scalia passed away in early 2016.

Gorsuch’s first call after being announced went to another judge named Merrick Garland, who’d been nominated by President Barack Obama after Scalia’s death. But in the midst of a heated presidential election, Republicans refused to consider Garland’s nomination and kept the seat empty until the next president was sworn in.

It was a risky move and Democrats were furious, but it ultimately paid off for Republicans when Trump won in a surprise victory last November.

Fast forward several months, Democrats were still steaming over what some have called a “stolen” Supreme Court seat and brought it up multiple times throughout Gorsuch’s confirmation hearings and in Senate floor speeches.

On top of that, Democrats also took issue with Gorsuch’s performance at his hearings, saying he was evasive in his answers, and they zeroed in on his decisions in a few cases, painting him as far-right and out-of-the-mainstream.

Republicans, on the other hand, argue Gorsuch answered more than 20 hours of questions and was abiding by what’s known as the Ginsburg standard so as not to show his cards on how he’d rule in cases that may come before him.

Hitting back against the argument that he’s extreme, Republicans say Gorsuch sided with the majority in 99% of his opinions as a federal judge in the past decade, and the GOP said that of the 2,700 cases he has ruled on, 97% were decided unanimously.

Republicans, in fact, felt that Trump picked a relatively safe nominee and rallied behind Gorsuch, even as Democrats signaled early on that they would filibuster his nomination.

When Democrats held the majority, they used the nuclear option in 2013 to advance lower court nominees, much to the disapproval of Republicans.

Now that Republicans are in the majority, they’re citing that Democratic action as a precedent.

Both parties acknowledge the risks of the change. It essentially allows the majority party to clear future Supreme Court nominees with ease, so presidents could appoint more ideological nominees that wouldn’t require much, if any, bipartisan support

“This is going to be very bad,” Republican Sen. Lindsey Graham told reporters Monday. “If you want to get a judge on the court you better have a majority. So this is going to haunt the Senate, it is going to change the judiciary, and it’s so unnecessary.”

Some fight to avoid going nuclear

A small, bipartisan group of senators held informal conversations in recent weeks to avoid the looming showdown.

“A number of us discussed it,” Republican Sen. John McCain told CNN. “We all wanted to do something, but we couldn’t agree on common ground. A testimony of the polarization of the Senate.”

Sen. Chris Coons, D-Delaware, said Wednesday that he worked with more than a dozen senators from both sides of the aisle to try to craft an agreement that would ensure the nuclear option wouldn’t be used again for future Supreme Court nominees.

“We just could not get there,” Coons said on NPR.

While Coons has expressed disappointment in Gorsuch’s answers at his hearings, he also offered some criticism of his own party.

“I know many on the left think this filibuster is a great thing and are celebrating the opposition to Judge Gorsuch, but the reality is looking forward, I think we are going to be looking at a Senate where the ability to work across the aisle, the ability to reach any agreement, and the ability to slow down any future highly partisan Supreme Court nominee will be less and less,” he said.